1. Field of the Invention
The present invention relates to a method of etching cavities having different aspect ratios, and more particularly, to a method of etching by means of sacrificial patterns.
2. Description of the Prior Art
Etching processes are the most common semiconductor technologies. The purpose of an etching process is to partially remove unnecessary thin film or substrate so as to define required structures or patterns. In the course of an etching process, the thin film not covered by a mask layer, such as a photo resist pattern, is removed chemically (i.e. by using an etching solution) or physically and chemically (i.e. by using plasma), thereby forming devices.
As MEMS (micro electromechanical system) devices, which have more complicated structures than semiconductor devices (for example, the wafer must be etched through), are developed, the yield of the etching process is more important than ever. Especially when cavities (or through cavities) having different aspect ratios need to be formed in a single etching process, the etching yield is frequently diminished due to different aspect ratios in different cavities. This is referred to as the Aspect-Ratio Dependent Etching (ARDE) effect.
Please refer to FIG. 1 through FIG. 4. FIG. 1 through FIG. 4 are schematic diagrams illustrating a conventional method of forming cavities having different aspect ratios, where FIG. 1 is a top view of a substrate 10, and FIG. 2 through FIG. 4 are cross-sectional views of the substrate 10 along line 1–1′. As shown in FIG. 1 and FIG. 2, a substrate 10 is provided. Then an etching stop layer 12 is formed on the bottom surface of the substrate 10, and a photo resist pattern 14 is formed on the top surface of the substrate 10. The photo resist pattern 14 is used to define a first cavity predetermined region 16 and a second cavity predetermined region 18 in the substrate 10.
Next, as shown in FIG. 3, an etching process is performed to remove the substrate 10 not covered by the photo resist pattern 14 to form a first cavity 20 and a second cavity 22 in the first cavity predetermined region 16 and the second cavity predetermined region 18, respectively. The first cavity 20 and the second cavity 22 have different aspect ratios, and thus lead to etching rate divergences. As shown in FIG. 3, while the substrate 10 in the first cavity predetermined region 16 has been etched through and stopped in the etching stop layer 12, the substrate 10 in the second cavity predetermined region 18 is not completely etched due to a slower etching rate.
As shown in FIG. 4, due to the slower rate in the second cavity predetermined region 18, the etching process must be continued until the substrate 10 is etched through so as to form the second cavity 22. However, if the etching process continues, the substrate 10 in the first cavity predetermined region 16 will be over-etched. The over-etching will cause damage to the inner walls or generate an undercut 24, and thus will result in structural deviations in the MEMS device.
It can be seen that the conventional method of etching cavities having different aspect ratios suffers from over-etching problems due to unequal etching rates. In view of this disadvantage, a new method of etching cavities having different aspect ratios is demanded.